Ethics Aside, Is NSA's Spy Tool Efficient?

Reports about the National Security Agency's program to collect vast amounts of data on personal electronic communications have created an uproar about the implications for privacy. But some statisticians and security experts have raised another objection: As a terror-fighting tool, it is highly inefficient and has some serious downsides.

Their reasoning: Any automated approach to spotting something rare necessarily produces false positives. That means for every correctly identified target, many more alarms that go off...